A new world raid boss would be cool. Maybe even world raid bosses spread throughout the zones of all expansions. A giant Magnataur in Dragonblight or something, just for fun, or a super Fel Reaver in Hellfire for level 90 raids.

An 8th Sha, however, is impossible. There were only 7 heads of Y'shaarj, and there was a corresponding Sha for each head. That is only further solidified by the 7 Burdens of Shao-hao. And then, on top of that, the Sha are defeated along with Y'shaarj's heart (as far as we know), meaning that even the established Sha cannot manifest any more.

Seriously, once an expansion is over, it is over. There's a dead period with no new content, and guess what...you are not FORCED to keep playing if it's so incredibly unbearable to you that you'd need them spending time making small additions.

I prefer they spend all their energy and resources into making WoD happen rather than trying to put band aid fixes out there for people too bored to play but not clever enough to unsub and do something else with their time.